Touch/multi-touch systems are becoming popular, not just on mobile phones, but also in the PC environment. Both MS Windows (RTM) 7 and KDE 4.4 support multi-touch interfaces. There are various touch-screen technologies, such as resistive, capacitive and projected capacitive that are suitable for small screen devices. These technologies are difficult to scale up for larger screens that are found on PCs and optical systems are a more suitable choice.
Typically, these systems are mounted at two or more corners of a screen and have an LED to illuminate either the finger or retro-reflective screens, and a sensor to observe the image. From the image, it is possible to calculate the position of the finger. For (X,Y) co-ordinate output, at least two sensors are required.
As these sensors are placed inside the bezel of a LCD/LED/plasma or similar screen, they must be connected to a controller (processing means) via a long cable (up to 0.5 m on a 20″ (51 cm screen). To maintain EMC (electro-magnetic compatibility), the RF (radio frequency) emissions from this cable should be kept to a minimum. Cost restrictions prevent the use of expensive, shielded multi-core cables and communication methods such as LVDS (low voltage differential signaling), where 4 conductors are used—one pair for clock and another for data.
The solution currently employed is to use an analog output from the sensor device. This requires only one conductor to carry the data. Also, due to the relatively low spatial content in the image, there is no high frequency voltage swings in the output, making processing easier. Although this technique works well, existing sensor technologies are therefore analog throughout the data path—from pixel through to output. This has several disadvantages: A long thin sensor has a lot of stray capacitance on the output bus, which makes signal degradation difficult to avoid; It requires the use of analog multiplexers which are susceptible to signal degradation such as rise and fall time limitations, cross-talk from neighboring pixels, noise coupling from the rest of the circuitry etc.; It is difficult or expensive to provide on-chip signal processing, for example: Companding of data for better dynamic range, e.g. gamma correction, logarithmic compression or similar, Subtraction of ambient light levels, Defect correction, Thresholding, Image compression.
There is a need in the art to address one or more of the above issues.